1. Field of the Invention
This invention relates to an improved AC memory drive type self-shift type gas discharge panel, and specifically to a new type panel structure which is capable of preventing accidental erroneous discharges caused by distributed abnormal charges.
2. Description of the Prior Art
A self-shift type gas discharge panel sends the information written in the form of discharge spots from the writing end of shift channel to the other end, where a period of the discharge cell arrangement for shift is considered as one picture element, and provides stationary display by suspending the shift operation on specific discharge cell groups during such information sending. Various types of panel structures have been proposed. FIGS. 1 and 2 respectively show in plan view and in cross section along the line II-II' the electrode arrangement of the meander electrode type gas discharge panel proposed in the U.S. Pat. No. 4,190,788 by Yoshikawa et al assigned to the same assignee as the present invention. In this case, a couple of shift channels SC1 and SC2 are typically indicated. A couple of Y (row) electrode groups y1i and y2i (where i is a positive integer), which have a meander pattern and which are guided respectively to the common buses Y1, Y2, are alternately arranged on the lower substrate 1. A couple of X (column) electrode groups x1j and x2j (where j is a positive integer) are alternately arranged at the inside of the upper substrate 2 in such a manner as to face the Y electrodes and are connected respectively to the common buses X1, X2. These X and Y electrodes on respective substrates are configured to form the shift channels SC1 and SC2. Each electrode of the X electrode groups x1j and x2j is placed in such a positional relation as to extend across an adjacent pair of electrodes of the opposing Y electrode groups y1i and y2i, and the surface of each electrode is covered with the dielectric layer 3 on the respective substrates. In addition, the write electrodes W1 and W2 are provided in the respective channels adjacent to the right most electrode x11 of the X electrode group and facing the right end electrode y11 of the Y electrode group. The discharge cells ai, bi, ci and di, formed with the four groups with four phases using each electrode alternately as a common electrode while cycling through the combinations of the four electrode groups regularly and periodically, are defined in the gap 4 between said electrodes arranged face to face. The gap 4 is filled with gas for discharge. Thereby the discharge spot generated by the write discharge cell W can be shifted sequentially along the arrangement of discharge cells. A surface layer of magnesium oxide (MgO) may be formed on said dielectric layer 3 as required in order to protect said dielectric layer from sputtering at the time of discharge.
The operation for writing information into the first shift channel SC1 in said panel structure is explained hereunder. First of all, since the write pulse is applied in accordance with said information to the write electrode W1, the write discharge cell w generates the first discharge spot at the timing where the shift electrode y11 is grounded. At this time, the shift pulse is applied to the phase A discharge cells ai of the shift channel, so that the discharge spot spreads simultaneously to the first shift discharge cell a1 adjacent to the discharge cell w by means of the priming effect of said write discharge spot. The discharge spot appearing at the discharge cell a1 may be sequentially shifted to the other end of the shift channel SC1 by shifting between adjacent pairs of discharge cells a1. b1, b1. c1, c1. d1, . . . when the shift pulses are sequentially applied to the adjacent discharge cells in the respective combinations phase A. phase B, phase B. phase C, phase C. phase D, . . . . During this operation, an erase pulse is applied to the discharge cells which have completed the shifting of a discharge spot, and thereby the undesired discharge spots are erased. As a result, the content of said information is displayed on the first shift channel SC1.
As explained above, the self-shift type gas discharge panel performs writing of the discharge spot and shift operation in accordance with the input information, however, the panel having such structure has the undesirable problem that an accidental erroneous discharge occurs at the end of the shift channel as the shift operation is repeated. Such accidental discharge is not observed at all in the well known matrix type panel and is peculiar to the self-shift type panel. Such accidental discharge has interfered with the display operation by disturbing the information within the panel. This erroneous discharge operation is now briefly explained. It may appear as a group of discharge spots around a single discharge spot of display information or it may appear as a comparatively large light emitting pattern after a momentary flash.
The inventors of this invention investigated this problem peculiar to the self-shift type panel and found that an accidental discharge results from the distribution of the stored wall charges at the ends of the shift channel due to sequential shift of the discharge spot. Namely, as explained previously, the shift operation of the discharge spot is performed by making use of the priming effect between adjacent discharge cells, and this priming effect is based on the coupling of space charges and on the coupling of wall charges. Coupling of wall charges occurs between cells which transfer the discharge spot in such a manner that electrons (minus charges) are supplied and stored and between cells which receive the same spot in such a manner that ions (plus charges) are supplied and stored. For this reason, as the shift operation advances sequentially, electrons are gradually left as wall charges at the writing end of the shift channel in the form of an excess of electrons, while the other end of the shift channel has a lack of electrons, that is, positive ions. Polarization thus occurs in the shift channel. FIG. 3 indicates this distribution of charges. The horizontal axis represents the shift channel with the right end considered as the end for writing, while the vertical axis represents potential.
Therefore, when this distribution of wall charges becomes sufficiently large due to the repetition of the shift operation, the abnormal electric field resulting from such abnormal wall charges induces an avalanche phenomenon in combination with an external field such as generated by the shift voltage, etc. The above-mentioned abnormal discharge which thus occurs is not based on the input information.
This accidental erroneous discharge is particularly remarkable for the case of the so-called drive method by the wall charge transfer system where the coupling of wall charges is positively used for the shift operation, as indicated in U.S. Pat. No. 3,781,600 by Coleman et al, rather than for the case of the so-called drive method by the space charge coupling system where the coupling of space charges is positively used for the shift operation which is indicated in the U.S. Pat. No. 4,132,924 by Yamagushi et al. The causes of said accidental erroneous discharge will be explained in more detail by referring to the drive voltage waveforms in the Coleman et al drive method. Namely, FIG. 4 shows the write electrode terminal w1 and the drive voltage waveforms to be applied to the shift buses Y1, Y2, X1, and X2, and as well the write and shift period SP and the display period DP.
As is apparent from the drive voltage waveforms of FIG. 4, since a positive write voltage Vw is applied to the write electrode w1 and the write discharge occurs during the data writing period T.sub.O, the minus wall charges are formed on the dielectric layer 3 of the relevant write electrode and the plus wall charges are formed on the dielectric layer 3 of the facing shift electrode y11. In the succeeding shift operation, the plus wall charges are transferred by the voltage of the succeeding shift electrodes sequentially being dropped to the ground potential from the shift voltage V.sub.sh. As a result, the minus charges are left at the surface of the cells after the shift operation. As these write operations and shift operations are repeated, the residual charges are not accumulated so much in the intermediate shift discharge cells since most of the charges are neutralized and erased by every polarity inversion, but the cells corresponding to the write electrodes are negatively charged by accumulation of residual minus charges and the shift termination cells are positively charged due to the accumulation of the plus charges that have been transferred.
Such abnormal discharge can be prevented by providing a discharge function for the normally stored charge at the electrodes of both ends of the shift channel. For example, the gas discharge panel disclosed in the U.S. Pat. No. 3,781,600 employs the structure for disabling storage of charges by directly exposing the electrodes at both ends of each shift channel to the gas discharge space. However, if these exposed electrodes are used, the electrode material sputters by the ion impact during discharge or the electrode is oxidized in the baking process for the sealing material on the occasion of sealing the discharge gas space. At any rate, such method has a disadvantage that the operating life is not so long due to a change of discharge characteristic at the area near the relevant electrodes. In addition, this method also has a problem that the upper limit of the write voltage margin is lowered. Namely, when the write voltage is applied to the exposed write electrodes, a heavy current flows therein for a comparatively long period, and this discharge causes unwanted discharges on the adjacent shift discharge cells. Therefore, the upper limit of said write voltage must be kept as low as possible.